Pneumocystis carinii is a major opportunistic pathogen of immunocompromised patients. Because P. carinii cannot be reliably cultured, molecular approaches have been utilized to identify and characterize antigens of this organism. Recombinant antigens can then be used to examine host immune responses to P. carinii infections. We have an ongoing project to characterize the antigens of both rat and human P. carinii. We have previously purified the major surface glycoprotein (MSG) of both rat and human pneumocystis using HPLC. It is necessary to use P. carinii from both sources because the organisms are different species. Subsequently, we identified a number of clones from a cDNA library of rat P. carinii that contain genes encoding for the MSG. These clones are clearly related but not identical, demonstrating that multiple genes encode the MSG. We have continued studies to characterize potential antigens of P. carinii genes. We have cloned a number of human P. carinii MSG genes and have expressed a full-length MSG in two fragments. We developed an ELISA to examine antibody responses to these antigens, and have utilized it to examine sera from patients with or without HIV infection, and with or without a history of PCP, as well as sera from a variety of healthy conrols. In about 15% of healthy patients followed serially we have been able to document changes in antibody titers, suggesting that these individuals have developed reinfection or reactivation of P. carinii infection. We will continue these studies to better understand the epidemiology of P. carinii infection in humans. We have also identified the unique expression site of MSG in human P. carinii, and can now identify the MSG variants that are expressed in a patient with PCP. Within this expression site we have identified a region of tandem repeats that varies among different P. carinii isolates, and thus provides a new method for typing human P. carinii. The goal of this study is to better understand the pathogenesis of P. carinii pneumonia with the hope that we can use this information to control or prevent this disease.